Investment Securities Available For Sale - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,417,724	$ 829,738
Unrealized Gains	21,031	16,449
Unrealized Losses	(25,674)	(27,211)
Investment securities available for sale	1,413,081	818,976
Proceeds from sales of investment securities	107,100	208,200	$ 99,200
Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
Investment securities held in securitization	47,922	43,897
Total Agency ARMs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	93,331	114,180
Unrealized Gains	22	95
Unrealized Losses	(2,281)	(1,430)
Investment securities available for sale	91,072	112,845
Total Agency Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,059,532	339,310
Unrealized Gains	669	0
Unrealized Losses	(12,916)	(10,683)
Investment securities available for sale	1,047,285	328,627
Total Agency IOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	41,367	97,153
Unrealized Gains	271	2,260
Unrealized Losses	(10,459)	(14,522)
Investment securities available for sale	31,179	84,891
Total Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,194,230	550,643
Unrealized Gains	962	2,355
Unrealized Losses	(25,656)	(26,635)
Investment securities available for sale	1,169,536	526,363
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	100,291	162,220
Unrealized Gains	1,852	1,218
Unrealized Losses	(18)	(154)
Investment securities available for sale	102,125	163,284
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	2,920
Unrealized Gains	0	0
Unrealized Losses	0	(33)
Investment securities available for sale	0	2,887
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	123,203	113,955
Unrealized Gains	18,217	12,876
Unrealized Losses	0	(389)
Investment securities available for sale	141,420	126,442
CMBS \| Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
Investment securities held in securitization	47,900	43,900
Freddie Mac \| Total Agency ARMs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33,623	39,138
Unrealized Gains	16	24
Unrealized Losses	(852)	(528)
Investment securities available for sale	32,787	38,634
Freddie Mac \| Total Agency Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,804	26,338
Unrealized Gains	0	0
Unrealized Losses	(736)	(644)
Investment securities available for sale	20,068	25,694
Freddie Mac \| Total Agency IOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,436	19,768
Unrealized Gains	19	559
Unrealized Losses	(2,756)	(3,363)
Investment securities available for sale	5,699	16,964
Fannie Mae \| Total Agency ARMs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	54,958	69,031
Unrealized Gains	6	71
Unrealized Losses	(1,236)	(698)
Investment securities available for sale	53,728	68,404
Fannie Mae \| Total Agency Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,038,363	312,515
Unrealized Gains	669	0
Unrealized Losses	(12,174)	(10,035)
Investment securities available for sale	1,026,858	302,480
Fannie Mae \| Total Agency IOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,310	27,597
Unrealized Gains	22	478
Unrealized Losses	(2,989)	(4,777)
Investment securities available for sale	8,343	23,298
Ginnie Mae \| Total Agency ARMs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,750	6,011
Unrealized Gains	0	0
Unrealized Losses	(193)	(204)
Investment securities available for sale	4,557	5,807
Ginnie Mae \| Total Agency Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	365	457
Unrealized Gains	0	0
Unrealized Losses	(6)	(4)
Investment securities available for sale	359	453
Ginnie Mae \| Total Agency IOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,621	49,788
Unrealized Gains	230	1,223
Unrealized Losses	(4,714)	(6,382)
Investment securities available for sale	$ 17,137	$ 44,629

The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

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